[R] Sample covariance matrix in R
Bert Gunter
gunter.berton at gene.com
Thu Nov 18 23:25:33 CET 2010
perhaps ...
library("fortunes")
fortune("surgery")
Cheers,
Bert
On Thu, Nov 18, 2010 at 1:57 PM, Ben Bolker <bbolker at gmail.com> wrote:
> Alaios <alaios <at> yahoo.com> writes:
>
>>
>> Hello everyone.
>> I would like to find the sample covariance matrix using R.
>>
>> So far I read on the wikipedia what a sample_covariance is
>> http://en.wikipedia.org/wiki/Sample_covariance
>>
>> according to wikipedia one vector is
>> enough to calculate the sample covariance matrix.
>
> I'm sorry, where does it say that?? The expression given
> for the sample covariance is
>
> q_{ij}=\frac{1}{N-1}\sum_{k=1}^{N}
> \left( x_{ik}-\bar{x}_i \right) \left( x_{jk}-\bar{x}_j \right) .
>
> if N=1 this whole thing will go up in smoke: the denominator and
> numerator will both be zero.
>
>
>> In R I tried cov(myvector) and I get the reply that I need to
>> pass either two argument or one matrix with x,y
>> values .
>>
>> How can I find the sample covariance matrix?
>>
>
> You need more than one sample (!); even trying to do it with
> two samples would give you a mathematically well-defined but
> statistically awful estimate.
>
> I strongly suggest that you consult a statistics book or
> a local expert ...
>
> good luck
>
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--
Bert Gunter
Genentech Nonclinical Biostatistics
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